Air conditioning (“AC”) systems are the primary driver of summer electricity use and peak power demand in certain parts of the country, such as Texas, mostly due to the refrigerant compressor in the condenser unit, with residential home and commercial AC systems contributing significantly to the peak demand. The power demand for a residential AC compressor is on the order of kilowatts.
Due to the tightening capacity margins of the power grid, and the corresponding increase in prices during peak demand hours in the summer (usually in the afternoon and early evening), utilities are seeking methods of reducing power demand. Since air conditioning systems are a significant contributor to the peak load, reducing the air conditioning load could help alleviate the problem.
Air conditioning use typically aligns with peak demand, especially during hot summer months, leading to higher wholesale prices in the afternoon and early evening. Utilities selling power on a flat rate are at risk of losing money during these peaks if power is more expensive to generate or purchase on the wholesale market than the price they sell it to the customers. If utilities can reduce peak demand, they might avoid high peak-time costs. If the utility offers the homeowner/customer time-of-use or real-time pricing, it would be financially valuable to the electricity customer to reduce consumption during those time periods, since the electricity would be more expensive per kilowatt hour than during off-peak times. Additionally, if the overall energy consumption could be reduced, it would provide even more direct savings to the homeowner.
Thermal energy storage is a method pre-cooling a thermal mass or medium that will later be drawn upon to assist or replace the AC condenser. The concept of thermal storage is not new, and is in widespread use in large-scale AC systems for the commercial and industrial sectors. However, smaller thermal storage systems, such as for residential and small commercial applications, while available, are not widespread due to high costs relative to the costs of the AC system.
At the same time, many of the regions with high air conditioning demand also face, or are at risk of, water scarcity problems. A significant amount of water is used by buildings (both residential and commercial) for non-potable uses, including irrigation and flushing toilets. One way to help mitigate water use is to capture and store rainwater for use onsite. Traditionally, the price of municipal water in many areas has been so low that large-scale rainwater harvesting systems are not economically practical. However, as drought depletes water reserves, rates are going up and many cities are imposing strict water use standards, and rainwater systems are receiving renewed interest. Large rainwater collection systems could help reduce building water demands, and reduce reliance on regional reservoirs and municipal water treatment and conveyance systems.
Unfortunately, there is not currently a means for integrating a thermal storage system with an on-site water collection and storage system into a single combined system to reduce the overall cost and to reduce redundant components. Such a combined energy/water storage configuration may increase the cost effectiveness of both a thermal storage system and a rainwater collection system by means of shared costs, avoided energy and water expenses as well as improve grid reliability thereby potentially financially benefiting both the electricity/water customer and the electricity/water utilities.